Nearly every vehicle has a radio receiver for receiving broadcast radio frequency (RF) signals such as amplitude modulation (AM) and frequency modulation (FM) signals. Many vehicle radio receivers also receive radio data system (RDS) signals. Vehicles include automobiles, trucks, bases, and the like.
Most radio receivers use omni-directional antennas to address the almost constantly changing reception conditions in a moving vehicle. The reception quality of RF signals and especially RDS signals usually is impaired by the multipath distortion associated with these antennas. Multipath distortion typically is a localized effect resulting from the interaction between multiple signals from a transmitter. Some broadcast signals bounce or reflect off objects such as buildings, mountains, and the like. The bounced or reflected signal and the direct signal often are not in phase or synchronized when received by an omni-directional antenna. The data in these out-of-phase or unsynchronized signals are often scrambled or otherwise unusable by the radio receiver.
Many radio receivers have multiple antennas to reduce multipath distortion. The antennas are spaced apart on the vehicle. The radio receiver switches between the spaced antennas to lessen the impact of multipath distortion. The spacing of the antennas reduces the likelihood of the antennas experiencing multipath distortion events at the same time.
Some radio receivers mix the incoming RF signals from the antennas to reduce the multipath distortion. The mixture of the RF signal forms a combined signal. The proportion of each RF signal in the combined signal usually is responsive to the signal quality of each RF signal. Signal quality generally is related to various characteristics of an RF signal such as the signal strength, the signal noise, and the like. The radio receiver generates the audio signal in response to the combined signal. The radio receiver usually checks the signal quality of the separate RF signals along the same signal path where the receiver adjusts the proportions of the RF signals to create the combined signal.
Other radio receivers switch to alternate frequencies to reduce multipath distortion. The alternate frequencies transmit the identical audio program. The radio receiver usually monitors the alternate frequencies to determine whether an alternate frequency has better signal quality. The radio receiver switches to each alternate frequency momentarily and then switches back to the original frequency. Some radio receivers avoid the resulting switching effect in the audio signal by stopping the mixing of the incoming RF signals. These radio receivers use one RF signal to continue generation of the audio signal and another RF signal to monitor the alternate frequencies.